Sad though it may be, some Mac users are unfamiliar with, and somewhat afraid of the command line.
Stefan, you can look at the folder in the finder by using the menu item "Go > Go to Folder..." (Cmd-Shift-G). Enter "/usr/local/lib/pd/extra" in the box there and you should be fine.
On a related note: In 10.3 this option is removed in the open/save dialogs, and instead you simply begin typing the path and a sheet drops down. It took me a while to discover this.
In brief, I would recommend installing the following directories: /Library/Pd/Externals (populated with the .pd_darwin externals and libraries) /Library/Pd/Help (with the corresponding help files and pddp files--so don't overwrite the official ones) /Library/Pd/Internals (an alias to /usr/local/lib/pd) This requires adding to the -path and -helppath command line/.pdrc, but it makes sense given all that you've done to make pd double-clickable. Very nice work!
This is a far better solution on OSX compared to /usr/ since it actually is the way the OS is designed to be used. There's also a directory called Application Support/ that is expressly for this type of thing, and it is where Max, Adobe software, and other apps using the plugin format put 3rd party items.
cgc